Imaging optical system, and imaging apparatus incorporating the same

ABSTRACT

An imaging optical system including, from the object side to the image side, a first lens group having positive refracting power, an aperture stop, and a second lens group having positive power, in which the first lens group consists of three lens subgroups, having positive, negative and positive refracting powers, respectively, from the object side to the image side, with satisfaction of the following: 
       4&lt;( LTL+fB )/ fB &lt;15  (1)
 
       0.3&lt; D 12/IH&lt;4  (2)
 
     where fB is an on-axis distance, from an image side-surface in the second lens group to an image plane upon focusing at infinity, LTL is an on-axis distance from an object side-surface in the first lens group to the image side-surface in the second lens group, D12 is an on-axis length from an image side-surface in the first lens group to an object side-surface in the second lens group upon focusing at infinity, and IH is a maximum image height.

This application is a divisional of U.S. application Ser. No. 13/948,896filed on Jul. 23, 2013, and claims benefit of Japanese Application No.2012-189519 filed in Japan on Aug. 30, 2012, the contents of which areincorporated by this reference.

BACKGROUND OF THE INVENTION

The present invention relates to an imaging optical system used withtaking optical systems, etc., and further to an imaging apparatus suchas digital still cameras using an imaging optical system.

So far, an imaging optical system like the one set forth in PatentPublication 1 has been known as a large-aperture imaging optical system.

Patent Publication 1: JP(A) 2009-251398 SUMMARY OF THE INVENTION

In one embodiment, the invention provides an imaging optical systemcomprising:

in order from an object side to an image side,

a first lens group having positive refracting power,

an aperture stop, and

a second lens group having positive refracting power, wherein:

there is no lens group other than said first lens group and said secondlens group,

said first lens group consists of three lens subgroups: in order fromthe object side to the image side, a first lens subgroup having positiverefracting power, a second lens subgroup having negative refractingpower, and a third lens subgroup having positive refracting power,

upon focusing from an object at infinity to a near distance object, saidfirst lens group remains stationary, and second lens group moves to theobject side.

In one embodiment, the invention provides an imaging apparatuscomprising:

said imaging optical system, and

an imaging device located on an image side of said imaging opticalsystem.

Still other objects and advantages of the invention will in part beobvious and will in part be apparent form the specification.

The invention accordingly comprises the features of construction,combinations of elements, and arrangement of parts which will beexemplified in the construction hereinafter set forth, and the scope ofthe invention will be indicated in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the imaging optical system according toExample 1 of the invention.

FIG. 2 is a sectional view of the imaging optical system according toExample 2 of the invention.

FIGS. 3A-3D show a set of aberration diagrams for the imaging opticalsystem according to Example 1 at infinity.

FIGS. 3E-3H show a set of aberration diagrams for the imaging opticalsystem according to Example 1 at a transverse magnification of 1/85.

FIGS. 3I-3L show a set of aberration diagrams for the imaging opticalsystem according to Example 1 at an object image distance of 500 mm.

FIGS. 4A-4D show a set of aberration diagrams for the imaging opticalsystem according to Example 2 at infinity.

FIGS. 4E-4H show a set of aberration diagrams for the imaging opticalsystem according to Example 2 at a transverse magnification of 1/85.

FIGS. 4I-4L show a set of aberration diagrams for the imaging opticalsystem according to Example 2 at an object image distance of 500 mm.

FIG. 5 is a cross-sectional view in schematic of the construction of adigital camera according to one embodiment of the invention.

FIG. 6 is a front view in perspective of the outside appearance of adigital camera according to one embodiment of the invention.

FIG. 7 is a rear view in perspective of the outside appearance of adigital camera according to one embodiment of the invention.

FIG. 8 is a block diagram illustrative of the controls of a digitalcamera according to one embodiment of the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The imaging optical system set forth in Patent Publication 1 has a totalangle of view of about 30°, and as the refracting power of a lens groupon the image side with respect to the aperture stop gets stronger tomake the angle of view wide while keeping the back focus intact, thereare spherical aberrations and coma likely to occur. The imaging opticalsystem of Patent Publication 1 is also operable to move a plurality oflens groups for focusing, but the mechanism involved may likely getcomplicated.

In one embodiment, the invention provides an imaging optical system thatfacilitates aberration reductions while makes sure brightness and aproper angle of view. In one embodiment, the invention provides animaging optical system that allows for a simple structure of thefocusing mechanism. In one embodiment, the invention provides an imagingapparatus incorporating such an imaging optical system.

According to one aspect of the invention, there is an imaging opticalsystem provided, which is basically built up of, in order from an objectside to an image side,

a first lens group having positive refracting power, an aperture stop,and a second lens group having positive refracting power, wherein:

there is no lens group other than said first lens group and said secondlens group, and

said first lens group consists of three lens subgroups: in order fromthe object side to the image side, a first lens subgroup having positiverefracting power, a second lens subgroup having negative refractingpower, and a third lens subgroup having positive refracting power.

The aforesaid basic construction of the inventive imaging optical systemwherein the first and second lens groups, each having positiverefracting power, are located with the aperture stop interposed betweenthem works in favor of offering tradeoffs between increasing apertureand decreasing aberrations. The converging action of the first lensgroup helps keep the heights of rays through the second lens group low,so much so that even when brightness is ensured, the diameter of thesecond lens group could be kept small.

To ensure a back focus at a focal length enough to allow the total angleof view to be greater than 40°, it is preferable to concentrate positiverefracting power on the image side of the whole imaging optical system.At this time, if the refracting power of the second lens group isbrought up to concentrate positive refracting power on the image side,spherical aberrations and field curvature may likely be produced at thesecond lens group.

For this reason, the third lens subgroup having positive refractingpower is located just in front of the aperture stop with the consequencethat the negative refracting power that must be shared by the secondlens group could be alleviated, thereby reducing the aberrationsproduced. More specifically, the lens setup on the object side withrespect to the aperture stop is made up of, in order from the objectside to the image side, the positive, first lens subgroup, the negative,second lens subgroup, and the positive, third lens subgroup that holdsback the occurrence of various aberrations at the second lens group.This lens layout works in favor of reducing aberrations throughout theimaging optical system, making sure brightness, and so on.

Preferably, such a basic construction should satisfy the followingConditions (1) and (2):

4<(LTL+fB)/fB<15  (1)

0.3<D12/IH<4  (2)

where fB is an on-axis distance, as calculated on an air basis, from animage side-surface in said second lens group to an image plane uponfocusing at infinity,

LTL is an on-axis distance from an object side-surface in said firstlens group to the image side-surface in said second lens group,

D12 is an on-axis length from an image side-surface in said first lensgroup to an object side-surface in said second lens group upon focusingon an object at infinity, and

IH is the maximum image height.

Keeping the imaging optical system against being short of the lowerlimit value of Condition (1) thereby bringing the image side-surface inthe second lens group close to the image plane may work in favor ofreductions of aberrations by making sure a lens setup space for thesecond lens group. At the same time, the effective diameter of thesecond lens group may be made small, working in favor of offeringtradeoffs between ensuring brightness and reducing size.

Keeping the imaging optical system against exceeding the upper limitvalue of Condition (1) may help reduce a risk of a camera bodyinterfering with the imaging optical system when it is used as aninterchangeable lens.

Keeping the imaging optical system against being short of the lowerlimit value of Condition (2) may make sure a separation between thefirst and the second lens group, thereby making sure a space forreceiving the aperture stop mechanism.

Keeping the imaging optical system against exceeding the upper limitvalue of Condition (2) may make the effective diameter of the first lensgroup small, working in favor of offering tradeoffs between ensuringbrightness and reducing size.

Preferably, the imaging optical system operates such that, upon focusingfrom an infinite object to a near distance object, the first lens groupremains stationary and the second lens group moves toward the objectside.

As described above, the invention makes it possible to reduce theoccurrence of various aberrations at the second lens group. To this end,if the second lens group is set up as a lens group capable of movingupon focusing, there is then an imaging optical system achieved that isless susceptible of aberration fluctuations during focusing. The numberof lens groups that move upon focusing may also be curtailed, leading toenergy savings. In addition, an inner focus system having a constantfull length is achievable, working in favor of preventing ingress ofdust during focusing, and reducing noise leakage during focusing aswell.

Preferably, the imaging optical system should satisfy the followingCondition (3):

1.2<f1/f2<2.5  (3)

where f1 is the focal length of the first lens group, and

f2 is the focal length of the second lens group.

Defining the focal length ratio between the first lens group and thesecond lens group by virtue of Condition (3) may work more in favor ofholding back various aberrations throughout the imaging optical system.

Keeping the imaging optical system against falling short of the lowerlimit value of Condition (3) may make sure the refracting power of thesecond lens group, working in favor of taking hold of the back focus.

Keeping the imaging optical system against exceeding the upper limitvalue of Condition (3) may lead to keeping the refracting power of thesecond lens group small, leading to making sure the lens groups beforeand after the aperture stop has a symmetrical refracting power profile,and working in favor of correction of coma, etc. throughout the imagingoptical system when it has a large aperture.

According to the second aspect of the invention, there is an imagingoptical system provided, comprising: in order from an object side to animage side,

a first lens group having positive refracting power, an aperture stop,and a second lens group having positive refracting power, wherein:

there is no lens group other than said first lens group and said secondlens group,

said first lens group consists of three lens subgroups: in order fromthe object side to the image side, a first lens subgroup having positiverefracting power, a second lens subgroup having negative refractingpower, and a third lens subgroup having positive refracting power, and

upon focusing from an infinite object to a near distance object, thefirst lens group remains stationary, and the second lens group movestoward the object side, with satisfaction of the following Condition(3)):

1.2<f1/f2<2.5  (3)

where f1 is the focal length of the first lens group, and

f2 is the focal length of the second lens group.

As described above, the second aspect of the invention may work in favorof making sure the back focus, the angle of view, brightness and opticalperformance, and focusing as well.

Any one of the aforesaid imaging optical systems should preferablysatisfy one of the following requirements, or two or more thereof at thesame time.

It is preferable that the second lens group comprises, in order from theobject side to the image side, a fourth lens subgroup and a fifth lensgroup having positive refracting power, wherein:

the fourth lens subgroup includes a negative lens that is a lenspositioned on the most object side in the fourth lens subgroup, and

the fifth lens subgroup includes a plurality of positive lenses.

The second lens group on the image side with respect to the aperturestop has a negative lens located on the most object side and thepositive, fifth lens subgroup located on the image side, setting up anarrangement approximate to the Gauss type that works much more in favorof offering tradeoffs between large apertures and reduced aberrations.The second lens group has a generally positive refracting power, and iftwo or more positive lenses are located in the fifth lens subgroup, itmay then favor reductions of spherical aberrations that occur as theaperture grows large, and coma as well.

The fourth and the fifth lens subgroups should each preferably includean aspheric lens surface.

The second lens group may reduce spherical aberrations, astigmatism andcoma by itself. This may work much more in favor of offering tradeoffsbetween performance improvements throughout the imaging optical systemin a full-focus state and large apertures.

Preferably, the fourth lens subgroup should consist of one cemented lensincluding a negative lens and a positive lens.

If the negative lens is cemented to the positive lens while thediverging action of the negative lens is maintained, it may then work infavor of reductions of chromatic aberrations, less deterioration ofimages due to lens decentration, and size reductions.

Preferably, the fifth lens subgroup should consist of three lenscomponents: in order from the object side to the image side, a positivelens component, a positive lens component and a negative lens component.

The “lens component” here is understood to refer to a lens block wherethere are only two refractive surfaces in on-axis contact with air: anobject side-surface and an image side-surface.

This works in favor of making sure the fifth lens subgroup has positiverefracting power and reductions of aberrations.

Preferably, the third lens subgroup should include a negative lens and apositive lens located more on the image side than the negative lens.

This allows the third lens subgroup to be of the retro focus type madeup of the negative and the positive lens in order from the object side,taking hold of the back focus.

Preferably, the first lens subgroup should include an aspheric lenssurface.

This may work much more in favor of correction of spherical aberrationsand astigmatism occurring when the first lens subgroup has a largeraperture.

Preferably, the first lens subgroup should consist of one positive lenscomponent,

the second lens subgroup should consist of one negative lens component,and

the third lens subgroup should consist of one positive lens component.

The “lens component” here is understood to refer to a lens block wherethere are only two refractive surfaces in on-axis contact with air: anobject side-surface and an image side-surface.

This may work in favor of cost and size reductions.

The third lens subgroup should consist of one cemented lens including apositive lens and a negative lens.

This may work in favor of reductions of chromatic aberrations, lessdeterioration of images due to lens decentration, and size reductions.

Preferably, the imaging optical system should preferably satisfy atleast one of the following Conditions (4) and (5):

0.80<φ/f<1.5  (4)

40°<2×ω<70°  (5)

where φ is the maximum diameter of the entrance pupil of the imagingoptical system,

f is the focal length of the imaging optical system, and

ω is the maximum taking half angle of view of the imaging opticalsystem.

Keeping the imaging optical system against running short of the lowerlimit value of Condition (4) may lead to taking hold of sufficientbrightness, because the focal length of the imaging optical system maybe kept short while the maximum diameter of the entrance pupil ismaintained.

Keeping the imaging optical system against exceeding the upper limitvalue of Condition (4) may lead to size reductions of the imagingoptical system, because the maximum diameter of the entrance pupil iskept moderate.

As the imaging optical system is kept against falling short of the lowerlimit value of Condition (5) to take hold of the angle of view andagainst exceeding the upper limit value of Condition (5) to keep theangle of view moderate, it may make sure an angle of view in favor ofoffering tradeoffs between cost reductions, reductions of aberrations,and taking hold of brightness, leading to a downsizing of the imagingoptical system.

It is here to be noted that Conditions (4) and (5) may be separatelyspecified.

If any one of the aforesaid imaging optical systems is combined with animaging device located on the image side of the imaging optical systeminto an imaging apparatus, it is then possible to take images by theimaging optical system that works in favor of offering tradeoffs betweentaking hold of brightness and taking hold of optical performance.

Two or more of the aforesaid requirements should preferably be satisfiedat the same time.

Preferably, each condition should be reduced down as follows, becauseits function may be more reliable.

Of Condition (1), it is more preferable that the lower limit value isset at 5, and especially 6, and the upper limit value is set at 11, andespecially 7.

Of Condition (2), it is more preferable that the lower limit value isset at 0.5, and especially 0.7, and the upper limit value is set at 2.5,and especially 1.

Of Condition (3), it is more preferable that the lower limit value isset at 1.5, and especially 1.8, and the upper limit value is set at 2.4,and especially 2.3.

Of Condition (4), it is more preferable that the lower limit value isset at 0.85, and especially 0.90, and the upper limit value is set at1.3, and especially 1.1.

Of Condition (5), it is more preferable that the lower limit value isset at 42°, and especially 44°, and the upper limit value is set at 60°,and especially 50°.

With the invention, it is possible to provide an imaging optical systemthat facilitates reductions of aberrations while brightness and a properangle of view are maintained. It is also possible to provide an imagingoptical system that makes it easy to simplify the focusing mechanisminvolved. Further, it is possible to provide an imaging apparatusincorporating such an imaging optical system.

The inventive imaging optical system will now be explained withreference to the accompanying drawings.

The inventive imaging optical system basically comprises, in order fromthe object side to the image side, the first lens group G₁ of positiverefracting power, the aperture stop S, and the second lens group G₂ ofpositive refracting power, wherein there is no lens group other than thefirst G₁ and the second lens group G₂, and the first lens group G₁consists of three lens subgroups: in order from the object side to theimage side, the first lens subgroup G_(S1) of positive refracting power,the second lens subgroup G_(S2) of negative refracting power, and thethird lens subgroup G_(S3) of positive refracting power.

In both Examples 1 and 2 of the inventive imaging optical system, thesecond lens group alone moves to the object side for focusing frominfinity to near distances.

In Examples 1 and 2, a plane plate C just in front of the imaging planerepresents an optically equivalent plane-parallel plate that is apackage comprising a cover glass of the imaging device, a low-passfilter, an infrared cut filter and a dust removal filter, and a planeplate on the object side of the cover glass C is an infrared cut filterF. The capital letter I is indicative of the image plane.

FIG. 1 is a sectional view of the imaging optical system of Example 1.

As depicted in FIG. 1, the imaging optical system of Example 1 is builtup of, in order from the object side to the image side, the first lensgroup G₁ of positive refracting power, the aperture stop S, and thesecond lens group G₂ of positive refracting power.

The first lens group G₁ is made up of, in order from the object side tothe image side, the first lens subgroup G_(S1) of positive refractingpower, the second lens subgroup G_(S2) of negative refracting power, andthe third lens subgroup G_(S3) of positive refracting power.

The first lens subgroup G_(S1) consists of one double-convex positivelens L₁₁.

The second lens subgroup G_(S2) consists of one negative meniscus lensL₁₂ convex on its object side.

The third lens subgroup G_(S3) consists of a cemented lens SU₁₁ of adouble-concave negative lens L₁₃ and a double-convex positive lens L₁₄.

The second lens group G₂ is made up of, in order from the object side tothe image side, the fourth lens subgroup G_(S4) of positive refractingpower, and the fifth lens subgroup G_(S5) of negative refracting power.

The fourth lens subgroup GS4 consists of a cemented lens SU₂₁ of adouble-concave negative lens L₂₁ and a double-convex positive lens L₂₂.

The fifth lens subgroup G_(S5) consists of a double-convex positive lensL₂₃, a double-convex positive lens L₂₄, and a negative meniscus lens L₂₅convex on its object side.

Between the first G₁ and the second lens group G₂ there is the aperturestop S interposed.

A total of five aspheric surfaces are used: two at both surfaces r₁ andr₂ of the double-convex positive lens L₁₁ forming the first lenssubgroup G_(S1) in the first lens group G₁, one at the most imageside-surface r₁₁ of the cemented lens SU₂₁ forming the fourth lenssubgroup G_(S4) in the second lens group G₂, and two at both surfacesr₁₄ and r₁₅ of the image side double-convex positive lens L₂₄ in thefifth lens subgroup G_(S5) in the second lens group G₂.

FIG. 2 is a sectional view of the imaging optical system of Example 2.

As depicted in FIG. 2, the imaging optical system of Example 2 is builtup of, in order from the object side to the image side, the first lensgroup G₁ of positive refracting power, the aperture stop S, and thesecond lens group G₂ of positive refracting power.

The first lens group G₁ is made up of, in order from the object side tothe image side, the first lens subgroup G_(S1) of positive refractingpower, the second lens subgroup G_(S2) of negative refracting power, andthe third lens subgroup G_(S3) of positive refracting power.

The first lens subgroup G_(S1) consists of one positive meniscus lensL₁₁ convex on its image side.

The second lens subgroup G_(S2) consists of one negative meniscus lensL₁₂ convex on its object side.

The third lens subgroup G_(S3) consists of a cemented lens SU₁₁ of adouble-concave negative lens L₁₃ and a double-convex positive lens L₁₄.

The second lens group G₂ is made up of, in order from the object side tothe image side, the fourth lens subgroup G_(S4) of positive refractingpower and the fifth lens subgroup G_(S5) of negative refracting power.

The fourth lens subgroup G_(S4) consists of a cemented lens SU₂₁ of adouble-concave negative lens L₂₁ and a double-convex positive lens L₂₂.

The fifth lens subgroup G_(S5) consists of a double-convex positive lensL₂₃, a double-convex positive lens L₂₄ and a negative meniscus lens L₂₅convex on its object side.

A total of five aspheric surfaces are used: two at both surfaces r₁ andr₂ of the double-convex positive lens L₁₁ forming the first lenssubgroup G_(S1) in the first lens group G₁, one at the most image sider₁₁ of the cemented lens SU₂₁ forming the fourth lens subgroup G_(S4) inthe second lens group G₂, and two at both surfaces r₁₄ and r₁₅ of theimage side double-convex positive lens L₂₄ in the fifth lens subgroupG_(S5) in the second lens group G₂.

Set out below are an assortment of numeral data in Examples 1 and 2(surface data, a variety of data, focus data and the focal lengths ofthe respective lens groups).

The surface data here include the radius of curvature r and surfaceseparation d of the lens surface for each surface number, the d (587.6nm)-line refractive index nd of each lens (optical medium), and thed-line Abbe constant □d of each lens (optical medium). The radius ofcurvature r and surface separation d are given in mm. In the surfacedata, “∞” given in the radius-of-curvature column is indicative ofinfinity.

Aspheric surface data include data about aspheric lens surfaces. Supposehere that x is an optical axis with the proviso that the direction oftravel of light is taken as positive, and y is a direction orthogonal tothe optical axis. Then, aspheric surface shape is represented by thefollowing formula.

x = (y²/r)/[1 + {1 − (1 + K) ⋅ (y/r)²}^(1/2)] + A 4y⁴ + A 6y⁶ + A 8y⁸ + A 10y¹⁰…▫

In the aforesaid formula, r is the paraxial radius of curvature, K isthe conical coefficient, and F4, A6, A8 and A10 are the 4^(th)-,6^(th)-, 8^(th)-, and 10^(th)-order aspheric coefficients. Note herethat the small letter “e” is indicative of an exponent power with thefollowing numeral having 10 as a base. For instance, “1.0e−5” means“1.0×10⁻⁵”.

The focus data here include focal lengths, F-numbers (FNO), angles ofview 2□)(°), variable surface separations d, back focuses (in air), fulllengths (in air), and image heights, all given in mm except for theF-numbers and angles of view.

The effective imaging area of the imaging device is designed to have arectangular shape. The value of the maximum image height is the one inthe effective imaging area throughout Examples 1 and 2, and the halfangle of view □ is the one of rays arriving at the maximum image heightin that effective imaging area.

The focal length data on the respective lens groups are shown by thefocal lengths f1 and f2 of the respective lens groups, given in mm.

Numeral Example 1

Surface Data Surface Number r d nd νd  1(Aspheric) 201.143 2.08 1.7725049.60  2(Aspheric) −42.609 0.10  3 8.969 1.77 1.83481 42.73  4 6.4364.25  5 −13.214 0.80 1.59551 39.24  6 9.000 3.99 1.83481 42.73  7−14.935 0.40  8(Stop) ∞ D8  9 −8.000 0.50 1.74077 27.79 10 14.356 4.001.77377 47.17 11(Aspheric) −9.703 0.10 12 12.420 3.00 1.88300 40.76 13−56.605 0.61 14(Aspheric) 66.859 1.60 1.77377 47.17 15(Aspheric) −40.0430.10 16 17.768 0.80 1.75211 25.05 17 6.000 D17 18 ∞ 0.30 1.51633 64.1419 ∞ 0.50 20 ∞ 0.50 1.51633 64.14 21 ∞ 0.62 Image Plane ∞ AsphericCoefficient 1^(st) surface K = 0.000 A4 = −1.23767e−04 A6 = −6.24020e−07A8 = 1.52562e−08 2^(nd) surface K = 0.000 A4 = −8.62813e−05 A6 =−1.54319e−07 A8 = 1.34452e−08 11^(th) surface K = 0.000 A4 = 3.29284e−04A6 = −4.77699e−06 A8 = 5.58840e−08 14^(th) surface K = 0.000 A4 =5.34065e−04 A6 = −2.45866e−05 A8 = 2.32076e−07 15^(th) surface K = 0.000A4 = 6.34365e−04 A6 = −2.10055e−05 A8 = 2.07520e−07 A10 = 3.38694e−10Focus Data Object Image Transverse Distance Infinity Magnification 1/85500 mm D8 3.44 3.29 3.15 D17 3.54 3.69 3.83 Various Data Focal Length10.92 FNO. 1.00 Angle of View 2ω (°) 46.48 Image Height 4.63 fb (in air)5.19 Full Length (in air) 32.73 Focal lengths of the Lens Groups f127.42 f2 12.84

Numeral Example 2

Surface Data Surface Number r d nd νd  1(Aspheric) −1000.000 2.001.77250 49.60  2(Aspheric) −36.817 0.10  3 9.522 2.00 1.83481 42.73  46.701 4.02  5 −14.827 0.80 1.59551 39.24  6 9.000 4.02 1.83481 42.73  7−14.818 0.40  8(Stop) ∞ D8  9 −8.000 0.80 1.74077 27.79 10 15.623 4.001.77377 47.17 11(Aspheric) −9.653 0.20 12 11.560 3.00 1.88300 40.76 13−68.803 0.10 14(Aspheric) 38.580 1.60 1.77377 47.17 15(Aspheric) −90.2470.10 16 20.625 0.80 1.75211 25.05 17 6.049 D17 18 ∞ 0.30 1.51633 64.1419 ∞ 0.50 20 ∞ 0.50 1.51633 64.14 21 ∞ 0.62 Image Plane ∞ AsphericCoefficient 1^(st) surface K = 0.000 A4 = −1.28950e−04 A6 = −6.04265e−07A8 = 1.82841e−08 2^(nd) surface K = 0.000 A4 = −7.52687e−05 A6 =−3.16039e−07 A8 = 1.80123e−08 11^(th) surface K = 0.000 A4 = 3.42708e−04A6 = −5.42091e−06 A8 = 6.23910e−08 14^(th) surface K = 0.000 A4 =4.82612e−04 A6 = −1.88400e−05 A8 = 1.52772e−07 15^(th) surface K = 0.000A4 = 5.18759e−04 A6 = −1.12303e−05 A8 = −1.41020e−08 A10 = 2.09877e−09Focus Data Object Image Transverse Distance Infinity Magnification 1/85500 mm D8 3.44 3.29 3.15 D8 3.41 3.25 3.10 D17 3.73 3.89 4.04 VariousData Focal Length 10.92 FNO. 1.00 Angle of View 2ω (°) 47.18 ImageHeight 4.63 fb (in air) 5.38 Full Length (in air) 32.73 Focal lengths ofthe Lens Groups f1 25.46 f2 13.01

FIGS. 3 and 4 are sets of aberration diagrams for Examples 1 and 2 atinfinity A to D, a transverse magnification of 1/85 E to H, and anobject image distance of 500 mm I to L.

In those aberration diagrams, SA, AS, DT and CC stand for sphericalaberrations, astigmatisms, distortions and chromatic aberrations ofmagnification, respectively. Given are spherical aberrations SA at therespective wavelengths of 587.6 mm (d-line: a solid line), 435.8 nm(g-line: a one-dot chain line) and 656.3 nm (C-line: a broken line), andchromatic aberrations of magnification CC at the respective wavelengthsof 435.8 nm (g-line: a one-dot chain line) and 656.3 nm (C-line: abroken line) on a d-line basis. Astigmatisms DT are given with thesagittal image plane as a solid line and the meridional image plane as abroken line. Note here that FNO and FIY are indicative of an F-numberand the maximum image height, respectively.

Tabulated below are the values of the respective parameters andConditions (1) to (5) in Examples 1 and 2.

Example 1 Example 2 Parameter LTL 27.54 27.35 fB 5.19 5.38 D12 3.84 3.81IH 4.63 4.63 f1 27.42 25.46 f2 12.84 13.01 φ 10.92 10.92 f 10.92 10.92Condition (1) (LTL + fB)/fB 6.31 6.08 (2) D12/IH 0.83 0.82 (3) f1/f22.14 1.96 (4) φ/f 1.00 1.00 (5) 2 × ω (°) 46.48 47.18

FIG. 5 is a sectional view of a single-lens mirrorless camera that isone example of the imaging apparatus that uses the imaging opticalsystem according to a specific embodiment of the invention andincorporates a small-format CCD, CMOS or the like as an imaging device.In FIG. 5, reference numeral 31 is a single-lens mirrorless camera; 32is an imaging lens system received within a lens barrel; and 33 is alens barrel mount for making the imaging lens system 32 attachable to ordetachable from the single-lens mirrorless camera 31. For that lensmount, for instance, a screw or bayonet type mount may be used. Thebayonet type mount is here used. Reference numerals 34 and 35 are animaging device plane and a back monitor, respectively.

As the imaging lens system 32 in the thus assembled single-lensmirrorless camera 31, for instance, the zoom lenses of Examples 1 and 2that embody the present invention may be used.

FIGS. 6 and 7 are illustrative in conception of the inventive imagingapparatus in which the imaging optical system is built in animage-taking optical system 41. More specifically, FIG. 6 is a frontperspective view of the outside configuration of a digital camera 40 asthe imaging apparatus, and FIG. 7 is a rear perspective view of thesame.

In this embodiment, the digital camera 40 includes the image-takingoptical system 41 positioned on a taking optical path 42, a shutterbutton 45, a liquid crystal display monitor 47, and so on. As theshutter button 45 located on the upper portion of the digital camera 40is pressed down, it causes images to be taken through the image-takingoptical system 41, for instance, the lens system of Example 1. An objectimage formed through the image-taking optical system 41 is formed on theimaging device (photoelectric transformation plane) located in thevicinity of the imaging plane. The object image received on the imagingdevice is displayed as an electronic image on the liquid crystal displaymonitor 47 located on the back of the camera via processing means. Thetaken electronic images may be recorded in recording means.

FIG. 8 is a block diagram for the internal circuitry in the main part ofthe digital camera 40. In what follows, the aforesaid processing meansshown by 51 is made up of, typically, a CDS/ADC block 24, a temporarystorage memory 17, and an image processing block 18, and a storage means52 is made up of, typically, a storage medium block.

The digital camera 40 includes an operating block 12, a control block 13connected to the operating block 12, an imaging drive circuit 16 and atemporal storage memory 17 connected to the control signal output portof the control block 13 via buses 14 and 15, an image processing block18, a storage medium block 19, a display block 20, and a presetinformation storage memory block 21.

The temporal storage memory 17, image processing block 18, storagemedium block 19, display block 20 and preset storage memory block 21 aredesigned such that data are mutually entered in or produced out of themvia a bus 22, and the imaging drive circuit 16 is connected with the CCD49 and CDS/ADC block 24.

The operating block 12 is a circuit including various input buttons andswitches, through which event information entered (by a camera operator)from outside is notified to the control block. The control block 13 is acentral computing unit that is made up of typically a CPU and has abuilt-in program memory (not shown): it is a circuit that, according tothe program loaded in that program memory, has control over the digitalcamera 40.

The CCD 49 is an imaging device that is driven and controlled by theimaging drive circuit 16, and converts or transforms light quantity perpixel of the object image formed through the imaging optical system 41into electric signals that are in turn sent out to the CDS/ADC block 24.

The CDS/ADC block 24 is a circuit that amplifies electrical signalsentered from CCD 49 and subjects them to analog-to-digital conversion sothat image raw data (Bayer data: hereinafter called the RAW data)subjected only to amplification and digital conversion are sent out tothe temporal memory 17.

The temporal storage memory 17 is a buffer made up of typically anSDRAM: it is a memory device for temporal storage of the RAW dataproduced out of the CDS/ADC block 24. The image processing block 18 is acircuit that reads out the RAW data stored in the temporal storagememory 17 or the RAW data stored in the storage medium block 19 therebyelectrically implementing various forms of processing includingdistortion correction, based on an image quality parameter instructed bythe control block 13.

The storage medium block 19 detachably receives a card type or sticktype recording medium comprising typically a flash memory so that theRAW data transferred from the temporal memory 17 or image data processedat the image processing block 18 are recorded and held in that flashmemory.

The display block 20 includes the liquid crystal display monitor 47 todisplay the taken RAW data or image data, operating menus or the like onit. The preset information storage memory block 21 includes a ROMsub-block having various image quality parameters previously loaded init, and a RAM sub-block for storing an image quality parameter read outof that ROM sub-block by entering operation of the operating block 12.

The thus assembled digital camera 40, because the inventive zoom lens isused as the imaging optical system 41, may be used as a small-formatimaging apparatus suitable well fit for the taking of moving images.

While various embodiments of the invention have been explained, it is tobe understood that the present invention is never limited thereto, andembodiments comprising combinations of the essential requirements andlimitations are embraced in the category of the invention too.

What is claimed is:
 1. An imaging optical system comprising, in orderfrom an object side to an image side, a first lens group having positiverefracting power, an aperture stop, and a second lens group havingpositive refracting power, wherein: there is no lens group other thansaid first lens group and said second lens group, said first lens groupconsists of three lens subgroups: in order from the object side to theimage side, a first lens subgroup having positive refracting power, asecond lens subgroup having negative refracting power, and a third lenssubgroup having positive refracting power, upon focusing from an objectat infinity to a near distance object, said first lens group remainsstationary, and second lens group moves to the object side.
 2. Theimaging optical system as recited in claim 1, wherein: said second lensgroup consists of, in order from the object side to the image side, afourth lens subgroup and a fifth lens subgroup of positive refractingpower.
 3. The imaging optical system as recited in claim 2, wherein:said fourth lens subgroup includes a negative lens that is a lenspositioned on the most object side in said fourth lens subgroup.
 4. Theimaging optical system as recited in claim 2, wherein: said fifth lenssubgroup includes a plurality of positive lenses.
 5. The imaging opticalsystem as recited in claim 2, wherein: said fourth lens subgroup andsaid fifth lens subgroup each include an aspheric lens surface.
 6. Theimaging optical system as recited in claim 2, wherein: said fourth lenssubgroup consists of one cemented lens including a negative lens and apositive lens.
 7. The imaging optical system as recited in claim 1,wherein: said first lens subgroup includes an aspheric lens surface. 8.The imaging optical system as recited in claim 1, wherein: said firstlens subgroup consists of one positive lens component, said second lenssubgroup consists of one negative lens component, and said third lenssubgroup consists of one positive lens component, provided that the lenscomponent is a lens block having only two refractive surfaces in on-axiscontact with air: an object side-surface and an image side-surface. 9.The imaging optical system as recited in claim 1, wherein: said thirdlens subgroup consists of one cemented lens including a positive lensand a negative lens.
 10. The imaging optical system as recited in claim1, which further satisfies the following Condition (2):0.3<D12/IH<4  (2) where D12 is an on-axis length from an imageside-surface in said first lens group to an object side-surface in saidsecond lens group upon focusing on an object at infinity, and IH is amaximum image height.
 11. The imaging optical system as recited in claim1, which further satisfies the following Condition (1):4<(LTL+fB)/fB<15  (1) where fB is an on-axis distance, as calculated onan air basis, from an image side-surface in said second lens group to animage plane upon focusing on an object at infinity, LTL is an on-axisdistance from an object side-surface in said first lens group to theimage side-surface in said second lens group.
 12. The imaging opticalsystem as recited in claim 1, which further satisfies the followingCondition (3):1.2<f1/f2<2.5  (3) where f1 is a focal length of the first lens group,and f2 is a focal length of the second lens group.
 13. The imagingoptical system as recited in claim 2, wherein: said fifth lens subgroupconsists of three lens components: in order from the object side to theimage side, a positive lens component, a positive lens component and anegative lens component, provided that the lens component is a lensblock having only two refractive surfaces in on-axis contact with air:an object side-surface and an image side-surface.
 14. The imagingoptical system as recited in claim 1, wherein: said third lens subgroupincludes a negative lens, and a positive lens located more on the imageside than said negative lens.
 15. The imaging optical system as recitedin claim 1, which further satisfies the following Conditions (4) and(5):0.80<φ/f<1.5  (4)40°<2×ω<70°  (5) where φ is a maximum diameter of an entrance pupil ofsaid imaging optical system, f is a focal length of said imaging opticalsystem, and ω is a maximum taking half angle of view of said imagingoptical system.
 16. An imaging apparatus, comprising: the imagingoptical system as recited claim 1, and an imaging device located on theimage side of said imaging optical system.